A very large number of portable electronic devices are becoming available to people throughout the world. Better integrated circuits have greatly reduced the electric energy required to operate these devices and rechargeable batteries are available to power these devices. However, sometimes such recharging is inconvenient or impossible.
Thermoelectric generators are well known. These devices utilize the physics principal known as the Seebeck effect discovered in 1821. If two conductors of different materials such as copper and iron are joined at their ends forming two junctions, and one junction is held at a higher temperature than the other junction, a voltage difference will arise between the two junctions. Most thermoelectric generating devices currently in use today utilize semiconductor materials, such as bismuth telluride, which are good conductors of electricity but poor conductors of heat. These semiconductors are typically heavily doped to create an excess of electrons (n-type) or a deficiency of electrons (p-type). An n-type semiconductor will develop a negative charge on the cold side and a p-type semiconductor will develop a positive charge on the cold side. Since each element of a semiconductor thermoelectric device will produce only a few millivolts it is generally useful to arrange the elements in series so as to produce higher voltages. Several techniques have been developed for arranging the semiconductor elements in series in thermoelectric devices. One such method is to use a so-called eggcrate design where a small eggcrate-shaped structure made of insulating material separates the thermoelectric elements and permits the elements to be connected in series in an automated fabrication process to reduce the cost of fabricating these modules and improve reliability. Modules of this design are described in U.S. Pat. No. 5,892,656. That patent is incorporated herein by reference. Such modules (HZ-2) are commercially available from Hi-Z Corporation with offices in San Diego, Calif. The dimensions of the module are 1.15 inchesxc3x971.15 inchesxc3x97.20 inch, and the module comprises a 14xc3x9714 array of thermoelectric elements. With a 200xc2x0 C. (360xc2x0 F.) temperature difference, it will deliver an open circuit voltage of 6.6 volts and has a design operating range of 2.5 to 4.5 volts with an energy conversion efficiency of 5 %.
The heat required to be removed from a material to produce a phase change from a liquid to a solid is called the heat of fusion or latent heat. One of the best known examples is water that requires a removal of 334 joules/gram (144 BTU/lb.) to make the phase changed from water to ice. This is a reversible process that the same amount of heat must be added to go from the solid to the liquid state as must be removed to go from the liquid to the solid state.
Gomez, U.S. Pat. No. 4,251,291 discloses an electric generation system in which solar energy irradiates upon a latent heat storage device to enable the heat to be stored at a relatively constant temperature to serve as the source of heat for a thermoelectric generator. The Gomez device has limited solar applications.
It is known in the prior art to connect a rechargeable chemical battery to an electric circuit that is powering an electronic device. If the normal source of electricity fails, the charged chemical battery can provide electricity to the electronic device, for as long as the chemical battery is able to maintain its charge. However, there are significant problems with using a rechargeable chemical battery as a backup power source. For example, rechargeable chemical batteries tend to be very temperature sensitive and will operate poorly at high or low ambient temperatures. Also, rechargeable chemical batteries have a limited shelf life. For example, if left on the shelf too long the chemicals inside the battery can migrate causing battery degradation. Also, rechargeable chemical batteries can only be recharged a relatively small number of times before the battery is unable to hold any further charges.
What is needed is a more reliable backup source of energy.
The present invention provides a heat of fusion thermoelectric generator. A phase change material in the generator provides a thermal energy source and the thermal energy is converted into electric energy with a thermoelectric module. The phase change material is melted using an external source of energy. The phase change material is in thermal contact with a hot side heat sink. At least one thermoelectric module is disposed between and in thermal contact with the hot side heat sink and a cold side heat sink. Electric power is generated by the thermoelectric module from a temperature difference between the hot side heat sink and the cold side heat sink.